I have added AZW3 to the list of file extensions in Calibre2OPDS, but the catalog web page produced does not list the AZW3 file under "Download" for a book that has the AZW3 file present. Is there something else I need to do?

I don't know why the AZW3 format exists, is it better than MOBI? Why does Calibre convert AZW files to MOBI automatically, but not AZW3 files? What is the best strategy, convert all to MOBI, or also keep the AZW3?